Heating, ventilation and/or air conditioning (HVAC) systems are often used to control the comfort level within a building or other structure. Such HVAC systems typically include an HVAC controller that controls various HVAC components of the HVAC system in order to affect and/or control one or more environmental conditions within the building. The HVAC components can include, for example, a furnace and an air conditioner.
In forced air systems, the conditioned air is typically provided by a furnace and/or air conditioner through a plenum to a network of supply air ducts that distribute the conditioned air throughout the building. A network of return air ducts is often used to return air from the building back to the furnace and/or air conditioner. A blower is used to draw the return air through the return air ducts, and drive the return air through the furnace and/or air conditioner and into the supply air ducts via the plenum. In some cases, some of the air is replaced over time with fresh outside air, often through an energy recovery ventilator.
In a zoned system, conditioned air is delivered to each zone based on the heat load in that zone. Damper actuators are typically placed in the supply air ducts that feed each zone. By activating the damper actuators, the conditioned air may be delivered to only those zones that are calling for conditioned air. When multiple zones are serviced by a common blower, the pressure in the supply air duct can change dramatically depending on how many zones are calling for conditioned air. For example, if all of the zones are calling for conditioned air, the pressure in the supply ducts that are open may be lower than if only a single zone is calling for conditioned air. In some cases, a bypass damper may be placed in a bypass duct that extends between the supply duct (or the plenum) and the return air duct. This may allow some of the supply air to pass directly to the return air duct when the pressure in the plenum rises above a threshold value, such as when only a small number of zones are calling for conditioned air. Because the bypass damper may reduce the overall energy efficiency of the HVAC system, it is desirable for the bypass damper to only be opened when necessary (e.g. to help protect the HVAC equipment).